Can You Hear Me?
In 1896, Lord Rayleigh wrote in his book, The Theory of Sound (Strutt, 1896, p. 1) ‘The sensation of a sound is a thing sui generis, not comparable with any of our other senses. Directly or indirectly, all questions connected with this subject must come for decision to the ear, and from it there can be no appeal.’ Sir James Jeans concluded his book Science & Music (Cambridge University Press, 1937) with the sentence, ‘Students of evolution in the animal world tell us that the ear was the last of the sense organs to arrive; it is beyond question the most intricate and the most wonderful.’
The displacement of the ear drum when listening to the quietest perceivable sounds is around one-hundredth of the diameter of a hydrogen molecule, and even a tone of 1 kHz at a level of 70 dB still displaces the ear drum by less than one-millionth of an inch (0.025 micrometres). Add to all these points the fact that our pinnae (outer ears) are individual to each one of us, and one has a recipe for great variability in human auditory perception as a whole, because the ability to perceive these minute differences are so great.
In fact, if the ear was only about 10 dB more sensitive than it is, we would hear a permanent hiss of random noise, due to detection of the Brownian motion of the air molecules. Some people can detect pitch changes of as little as 1/25th of a semitone (as reported by Seashore). Clearly there is much variability in all of this from individual to individual, and one test carried out on 16 professionals at the Royal Opera, Vienna, showed a 10:1 variability in pitch sensitivity from the most sensitive to the least sensitive. What is more, ears all produce their own non-linear distortions, both in the form of harmonic and inter-modulation distortions. I had one good friend who liked music when played quietly, but at around 85 dB SPL she would put her hands to her ears and beg for it to be turned down. It appeared that above a certain level, her auditory system clipped, and at that point all hell broke loose inside her head.
Of course, we still cannot enter each other’s brains, so the argument about whether we all perceive the colour blue in the same way cannot be answered. Similarly we cannot know that we perceive what other people hear when listening to similar sounds under similar circumstances. We are, however, now capable of taking very accurate in-ear measurements, and the suggestion from the findings is that what arrives at the eardrums of different people is clearly not the same, whereas what enters different people’s eyes to all intents and purposes is the same. Of course, some people may be colour-blind, whilst others may be long-sighted, shortsighted, or may have one or a combination of numerous other sight anomalies. Nevertheless, what arrives at the eye, as the sensory organ of sight, is largely the same for all of us. However, if we take the tympanic membrane (the ear drum) as being the ‘front-end’ of our auditory system, no such commonality exists. Indeed, even if we extend our concept of the front-end to some arbitrary point at, or in front of, our pinnae, things would still not be the same from person to person because we all have different shapes and sizes of heads and hair styles. This inevitably means that the entrances to our ear canals are separated by different distances, and have different shapes and textures of objects between them. Given the additional diffraction and reflexion effects from our torsos, the answer to the question of whether we all ‘see the same blue’ in the auditory domain seems to be clearly ‘no’, because even what reaches our ear drums is individual to each of us, let alone how our brains perceive the sounds.
There is abundant evidence to suggest that many aspects of our hearing are common to almost all of us, and this implies that there is a certain amount of ‘hard-wiring’ in our brains which predisposes us towards perceiving certain sensations from certain stimuli. Nevertheless, this does not preclude the possibility that some aspects of our auditory perception may be inherited, and that there may be a degree of variability in these genetically influenced features. Aside from physical damage to our hearing system, there may also be cultural or environmental aspects of our lives which give rise to some of us developing different levels of acuteness in specific aspects of our hearing, or that some aspects may be learned from repeated exposure to certain stimuli.
It would really appear to be stretching our ideas of the evolutionary process beyond reason, though, to presume that the gene pairings which code our pinnae could somehow be linked to the gene pairings which code any variables in our auditory perception systems. Furthermore, it would seem totally unreasonable to expect that if any such links did exist, that they could function in such a way that one process could complement the other such that all our overall perceptions of sound were equal. In fact, back in the 1970s, experiments were carried out (which will be discussed in later paragraphs) which more or less conclusively prove that this could not be the case. Almost without doubt, we do not all hear sounds in the same way, and hence there will almost certainly always be a degree of subjectivity in the judgment and choice of studio monitoring systems. There will be an even greater degree of variability in our choice of domestic hi-fi loudspeakers, which tend to be used in much less acoustically controlled surroundings.
Philip Newell is an international consultant on acoustic design and former technical director of Virgin Records. He has over 40 years’ experience in the recording industry and has been involved in the design of several hundred studios, including the famous Manor and Townhouse Studios. He is also author of Project Studios, Recording Spaces and Studio Monitoring Design, and co-author of Loudspeakers, all published by Focal Press.